A huge variety of positioning devices have been developed in the past and are used in different technical domains. In the metrology domain most positioning devices are based on encoding principles, using optical and magnetic rotary or linear encoders. The maximal precision of these devices are of the order of 1/10 of a micron or 1/10′000 of a degree for angular encoders. In most of the encoders types the sensing element is assembled close to the encoder element which is fixed to the object of which one wants to determine the linear or angular position. In situations wherein the position of an object has to be determined from a certain distance, lasers or other light sources are used to determine the position. Optical triangulation or interferometry for example are techniques used in a huge variety of configurations. These systems have fundamental drawbacks. Either they are simple and the possible resolution is not very high, typically 0.1 micron at most, or the resolution is very high, typically 1 nm, and the system therefore is very complex, cumbersome and very expensive.
In several situations such as encountered in the case of assembly and handling robots, several serial encoders might be needed to identify the position of the extremity of a mobile arm of the system. By combining different encoders, the errors of these encoders are added and the measurement precision is rarely better than half a micron. Also, it is not always evident to combine linear and rotational encoders in the same space allocated to perform the 3D position or movement of an object.
There exists in different technical fields an important need for a system that can measure with very high precision, typically sub nanometer, the 3D position and rotation of a moving element located at distances from the mm range up to the km range. It would be desirable that such a system would be compact, easy to install, cheap and also that the same concept can be used for relatively small distances, typically between 1 cm to several meters, up to very high distances greater than 1 km.
Masa et al. (patent PCT/EP2011/062104) discloses a system, called a shadow imager, which is based on the projection and analysis of a shadow. The system described in PCT/EP2011/062104 is inexpensive, easy to implement in a wide variety of geometries of positioning devices, and it allows to determine the change of the position of the light source with very high precision. It comprises one imaging device, typically a CMOS sensor, composed of an array of pixels sensitive to electromagnetic radiation, an optical mask composed of opaque and transparent regions and a light source, arranged so that the incident light on the optical mask projects a shadow on the sensor. The optical mask in the device described in PCT/EP2011/062104 comprises two optical patterns that act as an absolute a relative code in the shadow imaging system. As the light source can be attached to a moving element, the position of this moving element can also be determined with very high precision. The very high precision that can be obtained with a shadow imaging device is due to the fact that the repetitive pattern integrated on the mask is projected over the whole sensor, allowing as such much higher precisions than what can be obtained by the direct projection of a light spot on a detector array. In the case of a shadow imager it is also important to recognize that the shadow imaging detection method is insensitive to the light intensity. This fact leads to another big advantage for an optical positioning measurement system based on a shadow imaging system, because the intensity stability of the light source is not important, to the contrary of other systems based on direct light illumination on a detector array. The shadow imager can be constructed with different types of light sources, typically small lamps or preferably LED's and semiconductor lasers which can be easily fixed to moving elements. Also, as described in detail in PCT/EP2011/062104, by using several light sources and several imagers, one may realize configurations that allow to determine the 6 degrees of freedom of a moving element, i.e. the 3 cartesian coordinates and the 3 rotation angles.
It is an object of the invention to add an important improvement to systems based on a shadow imaging position measurement system.